In recent years, ubiquitous networks using wireless local and personal areas communication system such as wireless tags, Bluetooth (registered trademark), and ZigBee (registered trademark) have begun to be widely used in the fields of equipment control, traffic, distribution, environmental protection, food industry, agriculture, earthquake monitoring, health care and so on. With the development of applications or services, the number of users of the networks is expected to increase in the future. Here, a global ubiquitous network that allows several applications or services to be provided to a greater number of users and enlarges the service area has attracted considerable attention.
The network shown in FIG. 1 includes a base station 2 connected to a wired network 1, and a number of wireless terminals 3-1, 3-2 to 3-N (N is an integer) scattered over a wide area. The wireless terminals 3-1, 3-2 to 3-N are directly accommodated in the base station 2. In FIG. 1, the wireless terminals 3-1 and 3-2 establish one wireless link 5 with the base station 2, respectively. Further, the wireless terminal 3-N establishes two wireless links 5 with the base station 2.
Further, a plurality of base stations 2 may be connected to the wired network 1.
The wireless terminals 3-1, 3-2 to 3-N in the present network are driven by a battery. The wireless terminals 3-1, 3-2 to 3-N are low-power and low-performance terminals having only a minimum number of functions such as data measurement and transmission of measured data. Traffic from the wireless terminals 3-1, 3-2 to 3-N is characterized by (1-1) a small amount of data, (1-2) a relatively long transmission interval, and (1-3) high-periodicity for data generation. A number of such wireless terminals 3-1, 3-2 to 3-N are under one base station 2. Accordingly, traffic properties include a great amount of periodicity traffic of UL (data transmission from the wireless terminals 3-1, 3-2 to 3-N to the base station 2 at the side of the wired network 1), and generally increase in the total amount of traffic. Further, in the present network, one base station 2 must efficiently accommodate as many wireless terminals 3-1, 3-2 to 3-N as possible in order to collect as much data as possible from many wireless terminals 3-1, 3-2 to 3-N. Accordingly, such a network needs a media access control (MAC) protocol capable of realizing high throughput and a short transmission delay time while one base station efficiently accommodates a number of low-performance wireless terminals 3-1, 3-2 to 3-N.
Dynamic slot assignment (DSA), which is a centralized control method having high resource utilization efficiency, has been known as a MAC protocol satisfying the above requirement. In this method, the base station 2 dynamically assigns a bandwidth (slot) according to a request from the wireless terminals 3-1, 3-2 to 3-N.
FIG. 2 shows an example of a MAC frame in time division multiple access-time division duplex (TDMA-TDD). The MAC frame has a constant length and is divided into an uplink period and a downlink period. The downlink period consists of a broadcast area (referred to as a period) and a demand assignment area. The uplink period consists of a demand assignment (DA) area and a random access (RA) area.
The demand assignment area is a bandwidth assignment area for each of the wireless terminals 3-1, 3-2 to 3-N or the wireless links, and can be accessed without contention. Meanwhile, the random access area is used by the plurality of wireless terminals 3-1, 3-2 to 3-N using random access and is a contention-based access area. A broadcast control channel (Bch), a random access feedback channel (RFch), a frame control channel (Fch), a control channel (Cch), a data channel (Dch), and a random access channel (Rch) are used to transmit and receive data or control information. Bch is provided for frame synchronization and is used to report attribute information (e.g., a base station identifier (ID)) of the base station 2 to the wireless terminals 3-1, 3-2 to 3-N. Fch is used to notify of information about the bandwidth assignment for each of the wireless terminals 3-1, 3-2 to 3-N or the wireless links (e.g., an ID for specifying the wireless terminals 3-1, 3-2 to 3-N or the wireless links 5 assigned to the bandwidth, an assignment channel, an assignment position, and an assignment amount) in the demand assignment area in which the bandwidth assignment is performed in units of the wireless terminals 3-1, 3-2 to 3-N or the wireless links. RFch is used to notify of random access information (e.g., a random access result of a previous frame, a random access parameter (an initial window size (IWS) and a persistent factor (PF)), a start position of random access in the present frame, and the number of random access slots). Cch is used to transmit and receive control information for each wireless terminal or each wireless link, such as a bandwidth request (resource request, RREQ) or automatic repeat request (ARQ). Dch is used to transmit and receive data. Rch is a channel for random access and is used for wireless terminals 3-1, 3-2 to 3-N or the wireless links to transmit, by random access, RREQ to the base station 2.
In a DSA method, a random access is mainly employed for a wireless terminal to request a bandwidth because it can accommodate aperiodic, bursty data flexibly and efficiently.
FIG. 3 shows an example of an uplink data transmission sequence using such a method. In the present example, the base station 2 sequentially transmits Bch, RFch, and Fch from the beginning of a MAC frame to the wireless terminals 3-1, 3-2 to 3-N.
The wireless terminals 3-1, 3-2 to 3-N under the base station 2 can recognize a start position of the random access in the frame and the number of random access slots by receiving RFch. When the wireless terminals 3-1, 3-2 to 3-N transmit data to the base station 2, the wireless terminals 3-1, 3-2 to 3-N transmit an RREQ (bandwidth request) including an ID for specifying the wireless terminals 3-1, 3-2 to 3-N or the wireless links 5 using Rch in order to request a bandwidth for data to transmit. In this case, the wireless terminals 3-1, 3-2 to 3-N voluntarily determine a back-off time, which is a transmission deferred time, based on an exponential back-off algorithm in order to avoid collision with Rch from other wireless terminals 3-1, 3-2 to 3-N. In the algorithm, a certain random value (integer) ranging from 0 to a window size (WS) is generated. The random value is used as the number of back-off slot. A time taken for the elapse of the number of back-off slot is a back-off time. Further, in first random access, IWS broadcasted using RFch is used as WS.
When the back-off time is completed, the wireless terminals 3-1, 3-2 to 3-N transmit RREQ to the base station 2 using an Rch slot immediately after completion of waiting. If there is a collision with Rch from the other wireless terminals 3-1, 3-2 to 3-N, RREQ is retransmitted by applying an exponential back-off algorithm. When the base station 2 correctly receives RREQ, the base station 2 notifies of successful RREQ reception using RFch of a next frame, and assigns Dch corresponding to a bandwidth request value from RREQ. Further, in the next frame assigned Dch, the base station 2 assigns Cch for ARQ (ARQ-ACK/ARQ-NACK) to transmit a Dch arrival acknowledgement to the wireless terminal 2.
When ARQ-ACK Cch is received, the wireless terminals 3-1, 3-2 to 3-N complete the data transmission process. On the other hand, when ARQ-NACK Cch is received, the wireless terminals 3-1, 3-2 to 3-N retransmit RREQ by applying the exponential back-off algorithm.
In the dynamic slot assignment (DSA) using the random access, particularly, there are a number of wireless terminals 3-1, 3-2 to 3-N under the base station 2. Accordingly, the access to the random access (RA) increases and the RcH (random access channel) is highly likely to collide with another Rch, which increases overhead. The overhead is a transmission deferred time based on the exponential back-off algorithm, which deteriorates throughput and delay properties.    Non-patent Document 1: Ohta Atsushi, Nuno Fusao, Mochizuki Nobuaki, et al., “Development of 5 GHz bandwidth advanced wireless access (AWA) system-MAC/DCL function”, 2000 IEICE Society Conference B-5-39, pp. 327.